Modification of animal genomes has multiple uses. Where a gene is modified, by, for example, silencing the gene, prohibiting the expression of protein, changing expression product or expression levels, the function of the gene or region of the gene becomes apparent. Changes to the gene may also result in modification to a more desired phenotype. Treatment of disease is also possible, where defective genes can be modified. Effective gene targeting of animals and in particular large domestic animals in which the pronuclei are difficult to visually identify in a fertilized zygote has numerous challenges preventing reliable delivery of targeting constructs in order to effectively target a specific region in such animal genome.